Field of the Invention
The present invention relates to a method of forming a thin film and, more particularly, to a method of forming a thin film that eliminates air bubbles.
Description of the Related Art
Spin coating is a well-known method of forming a thin film on a semiconductor wafer. In one common approach to spin coating, a puddle of fluid resin is formed at the center of a semiconductor wafer, which is then spun at a high speed, e.g., 1500-4000 rpm. The centrifugal force causes the fluid resin at the center to spread out over the top surface of the wafer. The final thickness and uniformity of the film are typically defined by the rotational speed of the wafer and the length of time the wafer is rotated.
The fluid resin can be dispensed statically or dynamically. With a static dispense, the fluid resin is deposited onto the wafer while the wafer is stationary. With a dynamic dispense, the fluid resin is deposited onto the wafer while the wafer is spinning at a relatively moderate speed, e.g., 500 rpm.
One problem with conventional spin coating is that when a high-viscosity fluid resin is direct line dispensed, air bubbles can be present when the fluid resin is initially dispensed onto the wafer. For example, it is difficult to dispense a fluid resin with a viscosity greater than 400 cP through a nozzle, which is an output structure with an opening that restricts the flow of resin out of a supply line. As a result, the nozzle is often removed from the supply line so that the high-viscosity fluid resin can be dispensed directly from the supply line. Dispensing a fluid resin directly from a supply line is known as a direct line dispense.
One drawback of a direct line dispense is that, in-between uses, air can enter the supply line which, in turn, causes air bubbles to be present the next time the fluid resin is initially dispensed onto a wafer. When the wafer is then spun at a high speed, the air bubbles in the puddle of resin that lie away from the center of the wafer move towards and beyond the outer edge of the wafer in response to the centrifugal force of the high-speed spin.
However, the closer an air bubble lies to the center of the wafer, the less centrifugal force is present and the less likely it is that the air bubble will move beyond the outer edge of the wafer, thereby leaving an air bubble in the resin that remains on the wafer. Thus, there is a need for a method of forming a thin film with a high viscosity fluid resin that eliminates air bubbles which can be present when the fluid resin is initially dispensed.